\begin{tikzpicture}
    \tkzDefPoints{0/0/M, 3/0/N}
    \tkzDefPoints{0.5/1/P_1, 1.2/-1/P_2, 2/0/P_3}
    \tkzDefPointBy[projection= onto M--N](P_1)  \tkzGetPoint{P_1'}
    \tkzDefPointBy[projection= onto M--N](P_2)  \tkzGetPoint{P_2'}

    \tkzDrawSegment(M,N)
    \tkzDrawSegments[dashed](P_1,P_1'  P_2,P_2')
    \tkzDrawPoints(P_1, P_1', P_2, P_2', P_3)

    \tkzLabelPoints[left](M)
    \tkzLabelPoints[right](N)
    \tkzLabelPoints[above](P_1,P_2',P_3)
    \tkzLabelPoints[below](P_1',P_2)
    \tkzLabelPoint[below](P_3){$(P_3')$}
\end{tikzpicture}

